Note: Descriptions are shown in the official language in which they were submitted.
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Description
Title of the Invention: UREA MANUFACTURING METHOD
Field of the Invention
[0001]
The present invention relates to a urea
manufacturing method.
Background of the Invention
[0002]
In urea manufacturing plants, highly corrosive
ammonium carbamate is produced as an intermediate during
processes of synthesizing urea from ammonia and carbon
dioxide. Therefore, corrosion resistance is required of
various processing units and lines of the plants.
[0003]
JP-B 3987607 discloses inventions of a urea
synthesis method and a urea synthesis apparatus and
explains that corrosion preventive air is introduced into
a condenser, a synthesis column and a stripper (see
paragraphs 0028, 0046, 0055 and 0070).
[0004]
WO-A 2014-192823 discloses an invention of a urea
synthesis method. It explains that, in a urea synthesis
apparatus for performing a urea synthesis method, at
least some of portions at which a urea synthesis tower A,
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a stripper B and a condenser C, and piping connecting
them come in contact with corrosive fluids may be made of
an austenite-ferrite duplex stainless steel of a
particular composition, and in addition, in piping,
valves and the like, S31603 series general-purpose
stainless steel may also be used according to corrosion
environments. WO-A 2014-192823 explains that an amount
of corrosion preventive oxygen fed may be reduced, inert
gases are reduced, and a reaction yield is enhanced
(Effects of the Invention).
Summary of the Invention
[0005]
An object of the present invention is to provide a
urea manufacturing method capable of enhancing a reaction
yield of urea by inhibiting corrosion of processing units
and lines of a urea plant when manufacturing urea by the
plant.
[0006]
The present invention provides a method for
manufacturing urea from manufacturing raw materials
including ammonia and carbon dioxide in a urea
manufacturing plant,
wherein the urea manufacturing plant includes a
plurality of processing units including a reactor, a
stripper and a condenser, and a plurality of lines
connecting the plurality of processing units, and
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the inner wall surfaces of the plurality of
processing units and the plurality of lines are made of a
stainless steel, and at least some of the plurality of
lines is made of an austenitic stainless steel,
the urea manufacturing method including: forming a
passivation film on the inner wall surfaces of the
plurality of processing units and the plurality of lines
by supplying carbon dioxide of the manufacturing raw
material with added oxygen; continuously measuring a wall
thickness of the line made of the austenitic stainless
steel; and adjusting a supply amount of the oxygen in
response to a measurement value of the wall thickness to
control a corrosion rate and a reaction yield of urea (a
control method (A)).
[0007]
In addition, the present invention provides a
method for manufacturing urea from manufacturing raw
materials including ammonia and carbon dioxide in a urea
manufacturing plant,
wherein the urea manufacturing plant includes a
plurality of processing units including a reactor, a
stripper and a condenser, and a plurality of lines
connecting the plurality of processing units, and
the inner wall surfaces of the plurality of
processing units and the plurality of lines are made of a
stainless steel, and at least some of the plurality of
lines is made of an austenitic stainless steel,
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the urea manufacturing method including: forming a
passivation film on the inner wall surfaces of the
plurality of processing units and the plurality of lines
by supplying carbon dioxide of the manufacturing raw
material with added oxygen; measuring a concentration of
iron, chromium or nickel dissolved in urea or ammonia and
an operating temperature; and adjusting a supply amount
of the oxygen in response to measurement values of the
concentration and the operating temperature to control a
corrosion rate and a reaction yield of urea (a control
method (B)).
[0008]
Further, the present invention provides a method
for manufacturing urea from manufacturing raw materials
including ammonia and carbon dioxide in a urea
manufacturing plant,
wherein the urea manufacturing plant includes a
plurality of processing units including:
a reactor to produce a urea synthesis liquid using
carbon dioxide and ammonia as raw materials;
a stripper to decompose ammonium carbamate and
separate a mixed gas including ammonia and carbon dioxide
from the urea synthesis liquid produced in the reactor by
heating the urea synthesis liquid; and
a condenser to condense at least a portion of the
mixed gas obtained in the stripper by absorption onto an
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absorption medium and to generate a low-pressure steam
using heat generated at the time of the condensation, and
a plurality of lines connecting the plurality of
processing units, and
the inner wall surfaces of the plurality of
processing units and the plurality of lines are made of a
stainless steel, and at least some of the plurality of
lines is made of an austenitic stainless steel,
the urea manufacturing method including performing
any one or any two or three of the following control
methods (A) to (C):
(A) a control method in which, in the urea
manufacturing method, a passivation film is formed on the
inner wall surfaces of the plurality of processing units
and the plurality of lines by supplying carbon dioxide of
the manufacturing raw material with added oxygen, a wall
thickness of the line made of the austenitic stainless
steel is continuously measured, and a supply amount of
the oxygen is adjusted in response to a measurement value
of the wall thickness to control a corrosion rate and a
reaction yield of urea;
(B) a control method in which a concentration of
iron, chromium or nickel dissolved in urea or ammonia and
an operating temperature are measured, and a supply
amount of the oxygen is adjusted in response to
measurement values of the concentration and the operating
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temperature to control a corrosion rate and a reaction
yield of urea; and
(C) a control method in which operating pressures of
the plurality of processing units and their respective
operating temperatures, a flow rate of carbon dioxide
introduced as the raw material, an oxygen amount in the
raw material carbon dioxide and a flow rate of ammonia
introduced as the raw material are measured to calculate
respective corrosion rates of the plurality of processing
units and corrosion rates of the plurality of lines
connecting the plurality of processing units, and a
supply amount of the oxygen is adjusted thereby to
control a corrosion rate and a reaction yield of urea.
[0009]
According to the urea manufacturing method of the
present invention, corrosion of processing units and
lines of urea manufacturing plants during urea
manufacturing processes can be inhibited and a yield of
urea can thereby be maintained.
Brief Description of Drawings
[0010]
[FIG. 1] A schematic view showing a urea manufacturing
flow in a urea manufacturing plant.
[FIG. 2] A diagram for illustrating one embodiment of a
urea manufacturing method using the urea manufacturing
plant.
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[FIG. 3] A plot indicating a difference in corrosion
rates between a pipe on which a passivation film layer is
formed and a pipe on which no passivation film layer is
formed in Example 1.
Embodiments of the Invention
[0011]
A urea manufacturing method of the present invention
is explained referencing Fig. 1. A urea manufacturing
plant shown in Fig. 1 is one embodiment for performing
the urea manufacturing method of the present invention
and it is not limited thereto.
[0012]
In addition, a urea manufacturing flow in the urea
plant shown in Fig. 1 itself is publicly known and
substantially the same as, for example, those shown in
Fig. 3 of JP-B 3987607 and Fig. 2 of WO-A 2014-192823. A
reactor 1, a stripper 2, a condenser 3, a heat exchanger
and an ejector 6 shown in Fig. 1 are the same as a urea
synthesis column A, a stripper C, a condenser B
(including a scrubber F), a heat exchanger D and an
ejector G shown in Fig. 3 of JP-B 3987607, respectively.
[0013]
A feature of the urea manufacturing method of the
present invention is to control, for example, when urea
is manufactured in the urea manufacturing plant shown in
Fig. 1, a corrosion rate and a reaction yield of urea by
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adjusting a supply amount of oxygen in response to
certain measurement values, and the manufacturing method
including specific manufacturing processes and reaction
conditions is not particularly limited.
[0014]
In the urea manufacturing method of the present
invention, urea may also be manufactured by, for example,
a manufacturing method using a urea manufacturing plant
shown in Fig. 3 of JP-B 3987607 and using the same
manufacturing processes and conditions as a manufacturing
method described in paragraphs 0052 to 0062 or Example 3,
or a manufacturing method using the same manufacturing
processes and conditions as a manufacturing method
described in paragraphs 0040 to 0048 or 0060 of WO-A
2014-192823.
[0015]
In the manufacturing flow example shown in Fig. 1,
ammonia is supplied from an ammonia supply line 10 to the
bottom of the reactor 1, and in parallel therewith,
carbon dioxide is supplied from carbon dioxide supply
lines 11 and 11a to the bottom of the reactor 1, and they
are reacted inside the reactor 1 and a gas-liquid mixture
including urea is thereby obtained. The reactor 1 is a
unit to produce a urea synthesis liquid using carbon
dioxide and ammonia as raw materials.
[0016]
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The reactor 1 is made of, for example, carbon steel,
and on a portion corresponding to the inner wall surface,
a lining layer made of duplex stainless steel is formed.
Therefore, the wall thickness of the reactor 1 cannot be
measured with ultrasonic wall thickness gauges from the
outside.
[0017]
In the gas-liquid mixture obtained in the reactor 1,
urea, ammonium carbamate which is a reaction
intermediate, water and unreacted ammonia are present as
the liquid phase, and some unreacted ammonia, unreacted
carbon dioxide and inert gases are present as the gas
phase. The inert gases are impurities such as air
(oxygen) supplied for a corrosion protection purpose and
hydrogen included in the raw material carbon dioxide.
[0018]
Reaction conditions in the reactor 1 may be the same
as those in the case of using the urea manufacturing
plant shown in Fig. 3 of JP-B 3987607 as mentioned above,
and for example, it is preferable that a pressure be 130
to 250 bar (13,000 to 25,000 kPa), an N/C (a molar ratio
of ammonia and carbon dioxide) be 3.5 to 5.0, an H/C (a
molar ratio of water and carbon dioxide) be 1.0 or less,
a residence time be 10 to 40 minutes, and a temperature
be 180 to 200 C.
[0019]
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When carbon dioxide is supplied to the reactor 1, it
is pressurized by a compressor (which is connected to the
carbon dioxide supply lines 11 and 11a, although not
illustrated) as well as mixed with an adjustment amount
of oxygen. The oxygen may be pure oxygen or air. When
air is used, the air is preferably supplied through an
air filter or the like.
[0020]
Ammonia is, on its way to being supplied from the
ammonia supply line 10 to the reactor 1, preheated to
approximately 70 to 90 C via the heat exchanger 5 and
thereafter supplied to the reactor 1 together with
ammonia collected from the condenser 3 by the ejector 6.
[0021]
The gas-liquid mixture obtained in the reactor 1 is
delivered through a gas-liquid mixture line 12 to the top
of the stripper 2. The stripper 2 is a unit to separate
the mixed gas including unreacted ammonia and unreacted
carbon dioxide from the urea synthesis liquid produced in
the reactor 1 by heating the urea synthesis liquid.
[0022]
The stripper 2 is made of, for example, carbon
steel, and on a portion corresponding to the inner wall
surface, a lining layer made of duplex stainless steel is
formed. Therefore, the wall thickness of the stripper 2
cannot be measured with ultrasonic wall thickness gauges
from the outside.
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[0023]
From the bottom of the stripper 2, a carbon dioxide
gas serving as a stripping agent is supplied from the
carbon dioxide supply lines 11 and 11b. The stripper 2
is heated by a heater not illustrated so that the
internal temperature can be increased.
[0024]
Operating conditions in the stripper 2 may be the
same as those in the case of using the urea manufacturing
plant shown in Fig. 3 of JP-B 3987607 as mentioned above,
and for example, it is preferable that a pressure be 130
to 250 bar (13,000 to 25,000 kPa) and preferably 140 to
200 bar (14,000 to 20,000 kPa), and a temperature be 160
to 200 C.
[0025]
In the stripper 2, due to the heating and the
introduction of carbon dioxide serving as a stripping
agent, ammonium carbamate in the gas-liquid mixture
decomposes into ammonia and carbon dioxide to be
delivered through a returned gas line 14 to the bottom of
the condenser 3 as a high-temperature mixed gas of
unreacted ammonia, carbon dioxide, inert gases and water
(vapor).
[0026]
Urea, a trace amount of undecomposed ammonium
carbamate, unseparated ammonia, carbon dioxide and the
like in the gas-liquid mixture are collected through a
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urea collecting line 13 at the bottom of the stripper 2.
The urea collected through the urea collecting line 13 is
further subjected to a purification process in a
subsequent step (a low-pressure decomposition step) and
thereby enhanced in purity. The trace amount of residual
ammonium carbamate is subjected to a decomposition
process and thereby becomes a low-temperature recycle
liquid including ammonia and carbon dioxide (also
including unreacted ammonia and carbon dioxide) and is
delivered through a recycle line 17 to the top of the
condenser 3 (scrubber) as an absorption medium.
[0027]
The condenser 3 is a unit to condense at least a
portion of the mixed gas obtained in the stripper 2 by
absorption onto the absorption medium and to generate a
low-pressure steam using heat generated at the time of
the condensation. Ammonia included in the mixed gas of a
high-temperature state supplied to the bottom of the
condenser 3 is cooled and condensed, and thereafter
delivered through a down pipe 15 to the raw material
ammonia supply line 10 by a suctioning function of the
ejector 6, and recycled as a urea manufacturing material.
[0028]
Some of the ammonia, carbon dioxide and water
(vapor) accompanying the inert gases of a high-
temperature state supplied to the bottom of the condenser
3 come in contact with the absorption medium during a
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process of being cooled and discharged as a low-
temperature gas through an exhaust line 16, and the
ammonia and the carbon dioxide are thereby absorbed and
removed, and the inert gases are discharged through the
exhaust line 16.
[0029]
The condenser 3 is made of, for example, carbon
steel, and on a portion corresponding to the inner wall
surface, a lining layer made of duplex stainless steel is
formed. Therefore, the wall thickness of the condenser 3
cannot be measured with ultrasonic wall thickness gauges
from the outside.
[0030]
Cooling water is introduced from a cooling water
line 21 into the condenser 3 and heat-exchanged and
vaporized therewithin into vapor, which is gathered
through a vapor line 22 and recycled as high-temperature
vapor. Operating conditions in the condenser 3 may be
the same as those in the case of using the urea
manufacturing plant shown in Fig. 3 of JP-B 3987607 as
mentioned above, and for example, it is preferable that a
pressure be 140 to 250 bar (14,000 to 25,000 kPa), a
temperature be 130 to 250 C (preferably 170 to 190 C), an
N/C be 2.5 to 3.5, an H/C be 1.0 or less, and a residence
time be 10 to 30 minutes.
[0031]
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For each line mentioned above, a pipe of an
austenitic stainless steel (single phase) or a pipe of a
duplex stainless steel (austenite-ferrite duplex
stainless steel) may be used. However, in the example of
the urea plant shown in Fig. 1, each line with any of
wall thickness measurement parts 30 to 37 by ultrasonic
wall thickness gauges is made of the pipe of the
austenitic stainless steel.
[0032]
As the austenitic stainless steel, for example,
S31603 (316L SS) may be used, and as the duplex stainless
steel, for example, a 25Cr duplex stainless steel
(S31260) or a 28Cr duplex stainless steel (S32808: DP28W)
may be used. Since each line is made of a single
material, the wall thickness can be measured with
ultrasonic wall thickness gauges from the outside.
[0033]
It is known that, during the urea manufacturing
process in the urea manufacturing plant shown in Fig. 1,
highly metal-corrosive ammonium carbamate, which is a
byproduct of a reaction of ammonia and carbon dioxide,
corrodes the inner wall surfaces of the reactor 1, the
stripper 2 and the condenser 3 or corrodes the inner wall
surface of each line.
[0034]
In the manufacturing method of the present
invention, contact of the stainless steels with ammonium
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carbamate is inhibited by forming a passivation film on
the surfaces of the stainless steels by mixing oxygen
into the raw material carbon dioxide, and the corrosion
of the stainless steels is thereby inhibited. Note that
the austenitic stainless steel has a property of
requiring more oxygen compared to the duplex stainless
steels in order to form the passivation film. However, a
too-high oxygen concentration in the raw material carbon
dioxide cannot fully increase temperatures inside the
reactor 1 and inside the condenser 3 and cannot increase
reaction rates either and therefore causes a reduction in
a reaction yield of urea (a reduction in a yield), and a
too-low oxygen concentration excessively promotes the
corrosion of the stainless steels.
[0035]
Note that Fig. 5 of WO-A 2014-192823 indicates a
relation between an oxygen concentration in a gas phase
(the horizontal axis) and a corrosion rate (the vertical
axis). It suggests that, for the austenitic stainless
steel (S31603), a passivation film is hard to form
compared to the 25Cr duplex stainless steel (S31260) and
the 28Cr duplex stainless steel (S32808), so that, when
the oxygen concentration is low, the corrosion rate
becomes larger, and when the oxygen concentration becomes
higher, the corrosion rate becomes smaller because a
passivation film is formed on any of the stainless
steels.
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[0036]
In the manufacturing method of the present
invention, it is preferable that any one or any two or
three of the following control methods (A) to (C) be
performed.
[0037]
Control method (A)
A control method (A) is a control method in which,
in the urea manufacturing method, a passivation film is
formed on the inner wall surfaces of the plurality of
processing units (including the reactor 1, the stripper 2
and the condenser 3) and the plurality of lines by
supplying carbon dioxide of a manufacturing raw material
with added oxygen, a wall thickness of a line made of an
austenitic stainless steel is continuously measured, and
a supply amount of the oxygen is adjusted in response to
a measurement value of the wall thickness to control a
corrosion rate and a reaction yield of urea.
[0038]
In the example of the urea manufacturing plant shown
in Fig. 1, a thickness t2 of each line during operation
is continuously measured with ultrasonic wall thickness
gauges, for example, at the wall thickness measurement
parts 30 to 37, thereby determining a corrosion rate s
(mm/year) from the following formula: s=(t1-
t2)/(operating time) (t1 represents an initial thickness
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of each line at the wall thickness measurement parts 30
to 37 before operation).
[0039]
The initial thickness t1 of each line is known (a
measurement value or a specification value) and the
corrosion rate s is obtained by dividing a difference
between the initial thickness t1 and the thickness t2 of
each line after operation by an operating time.
Therefore, changes in the corrosion rate s can be
continuously checked by continuously measuring the
thickness t2 of each line during operation.
[0040]
Accordingly, as an example of the control method
(A), when the corrosion rate s becomes too high, the
oxygen supply amount (when air is used, an air amount in
terms of an oxygen amount) is increased, and when the
corrosion rate s is sufficiently small, the oxygen supply
amount is decreased. This makes it possible to inhibit
variations of increase and decrease in the reaction yield
of urea to be as small as possible so that urea can be
manufactured at a stable reaction yield.
[0041]
The corrosion rate s in each line at the time of
operating the urea manufacturing plant shown in Fig. 1 is
preferably controlled to be 0.2 mm/year or less and more
preferably controlled to be 0.15 mm/year or less in terms
of the relation between s and the reaction yield of urea.
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[0042]
Control method (B)
A control method (B) is a control method in which a
concentration of iron, chromium or nickel dissolved in
urea or ammonia and an operating temperature are
measured, and a supply amount of the oxygen is adjusted
in response to measurement values of the concentration
and the operating temperature to control a corrosion rate
and a reaction yield of urea.
[0043]
In the example of the urea manufacturing plant shown
in Fig. 1, sampling can be performed, for example, at
sampling positions 40 to 42. At the sampling position
40, along with sampling of, for example, the gas-liquid
mixture including urea, ammonium carbamate which is a
reaction intermediate and unreacted gases (ammonia and
carbon dioxide) flowing through the gas-liquid mixture
line 12, the temperature is measured, and each ion
concentration of iron, chromium or nickel in the sample
is thereafter measured.
[0044]
At the sampling position 41, along with sampling of,
for example, urea, a trace amount of ammonium carbamate
and the like flowing through the urea collecting line 13,
the temperature is measured, and the respective ion
concentrations of iron, chromium and nickel in the sample
are thereafter measured.
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[0045]
At the sampling position 42, along with sampling of,
for example, a liquid including ammonia flowing through
the down pipe 15, the temperature is measured, and each
ion concentration of iron, chromium or nickel in the
sample is thereafter measured.
[0046]
When a result of the measurement is that the
respective ion concentrations of iron, chromium and
nickel in the sample are high, it is assumed that the
formation of the passivation film is insufficient and the
corrosion is in progress, and when a result of the
measurement is that each ion concentration of iron,
chromium or nickel in the sample is low, it is assumed
that the formation of the passivation film is sufficient
and the corrosion is not in progress. The ion of iron,
chromium or nickel to be measured may be any one of, or a
combination of any two of, or all the three of them. In
addition, as a result of the measurement, high
temperatures at the sampling positions suggest that the
corrosion progress becomes fast, and low temperatures at
the sampling position suggest that the corrosion progress
becomes slow.
[0047]
When the control method (B) is performed, it is
preferable that the sampling be performed at a plurality
of locations in the urea manufacturing plant and
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operating temperatures be measured at the plurality of
sampling locations. The sampling locations (temperature
measurement locations) are not particularly limited and a
plurality of locations (preferably three or more
locations) can be selected. For example, the outlet-side
line of the reactor 1 (gas-liquid mixture line 12), the
outlet-side line of the stripper 2 (urea collecting line
13) and the outlet-side line of the condenser 3 (down
pipe 15) are preferable.
[0048]
Note that temperatures inside the units, i.e., the
reactor 1, the stripper 2 and the condenser 3 which are
each near the sampling locations are also preferably
measured as the operating temperatures. The temperature
measurement may be performed with publicly-known
thermometers such as thermocouples or temperature
measuring resistors.
[0049]
Thus, variations of increase and decrease in the
reaction yield of urea can be inhibited so that urea can
be manufactured at a stable reaction yield by performing
any of the followings as the control method (B):
when the concentrations of iron, chromium and nickel
are high and the temperatures at the sampling positions
are high, the oxygen supply amount is increased to form a
passivation film (a first form of the control method
(B) ) ;
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when the concentrations of iron, chromium and nickel
are low and the temperatures at the sampling positions
are low, the oxygen supply amount is decreased (a second
form of the control method (B));
when the concentrations of iron, chromium and nickel
are high and the temperatures at the sampling positions
are low, the oxygen supply amount is increased (however,
an increase amount is less than that of the first form)
to form a passivation film (a third form of the control
method (B)); and
when the concentrations of iron, chromium and nickel
are low and the temperatures at the sampling positions
are high, the oxygen supply amount is decreased (however,
a decrease amount is less than that of the second form)
(a fourth form of the control method (B)).
[0050]
Control method (C)
A control method (C) is a control method in which
operating pressures of the plurality of processing units
(the reactor, the stripper and the condenser) and their
respective operating temperatures, a flow rate of carbon
dioxide introduced as a raw material, an oxygen amount in
the raw material carbon dioxide and a flow rate of
ammonia introduced as a raw material are measured to
calculate respective corrosion rates of the plurality of
processing units and corrosion rates of the plurality of
lines connecting the plurality of processing units, and a
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supply amount of the oxygen is adjusted thereby to
control a corrosion rate and a reaction yield of urea.
[0051]
The operating temperature of the reactor 1 may be
measured, for example, at an upper (preferably near the
top) measurement part (measuring instrument) 51 or a
lower measurement part (measuring instrument) 54 of the
reactor 1. The operating temperature of the stripper 2
may be measured, for example, at an upper (preferably
near the top) measurement part (measuring instrument) 52
or a lower measurement part (measuring instrument) 55 of
the stripper 2. The operating temperature of the
condenser 3 may be measured, for example, at an upper
(preferably near the top) measurement part (measuring
instrument) 53 or a lower measurement part (measuring
instrument) 56 of the condenser 3.
[0052]
The reactor 1, the stripper 2 and the condenser 3
are approximately the same in pressure. These pressures
may be measured, for example, at the line llb or at an
ammonia injection line to the condenser 3 not
illustrated.
[0053]
The flow rate of carbon dioxide introduced as a raw
material may be measured, for example, at the carbon
dioxide supply lines 11 and 11a. When carbon dioxide is
supplied to the reactor 1, it is pressurized by a
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compressor as well as mixed with an adjustment amount of
oxygen. As such, an oxygen amount in the raw material
carbon dioxide may be calculated, for example, from an
amount of air introduced into the compressor. The flow
rate of ammonia introduced as a raw material may be
measured, for example, at the ammonia supply line 10.
[0054]
The respective corrosion rates of the reactor 1, the
stripper 2 and the condenser 3 and those of the plurality
of lines (the gas-liquid mixture line 12, the returned
gas line 14 and the down pipe 15) connecting the reactor
1, the stripper 2 and the condenser 3 may be determined
as follows from the aforementioned measurement data,
i.e., the operating temperatures, the operating
pressures, the flow rate of carbon dioxide, the oxygen
concentration in carbon dioxide and the flow rate of
ammonia. They may be determined considering the
followings on the basis of the relation between the
measurement data and the corrosion rate in the control
method (A): the higher the operating temperatures become,
the larger the corrosion rates become; the higher the
ammonium carbamate concentrations become, the larger the
corrosion rates become; and the higher the oxygen
concentration in carbon dioxide becomes, the smaller the
corrosion rates become.
[0055]
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A further preferable embodiment of the urea
manufacturing method of the present invention is
explained referencing Fig. 2. In the embodiment shown in
Fig. 2, the control methods (A), (B) and (C) are
performed in this order.
[0056]
In stage (1), urea manufacture, for example,
according to the manufacturing flow illustrated in Fig.
1, is started. After the start of urea manufacture, the
control methods (A) to (C) for controlling a corrosion
rate and a reaction yield of urea by adjusting a supply
amount of oxygen are performed.
[0057]
In stage (2), whether to increase the supply amount
of air (oxygen) in the raw material carbon dioxide or to
maintain the current amount is determined by the control
method (A). When the corrosion rate determined in the
control method (A) is within acceptable values (Yes), the
method proceeds to stage (3). When the corrosion rate
determined in the control method (A) exceeds an
acceptable value (No), the method proceeds to stage (5)
in order to enhance corrosion protective effect, and the
urea manufacture is continued while the supply amount of
air (oxygen) in the raw material carbon dioxide is
increased. In stage (2), if the method proceeds to stage
(5) and the supply amount of air (oxygen) in the raw
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material carbon dioxide is increased, stage (3) or later
is not performed.
[0058]
In stage (3), whether to increase the supply amount
of air (oxygen) in the raw material carbon dioxide or to
maintain the current amount is determined by the control
method (B). When the corrosion rate determined in the
control method (B) is within acceptable values (Yes), the
method proceeds to stage (4). When the corrosion rate
determined in the control method (B) exceeds an
acceptable value (No), the method proceeds to stage (5)
in order to enhance corrosion protective effect, and the
urea manufacture is continued while the supply amount of
air (oxygen) in the raw material carbon dioxide is
increased. In stage (3), if the method proceeds to stage
(5) and the supply amount of air (oxygen) in the raw
material carbon dioxide is increased, stage (4) or later
is not performed.
[0059]
In stage (4), whether to increase the supply amount
of air (oxygen) in the raw material carbon dioxide or to
maintain the current amount is determined by the control
method (C). When the corrosion rate determined in the
control method (C) is within acceptable values (Yes), the
method proceeds to stage (5). When the corrosion rate
determined in the control method (C) exceeds an
acceptable value (No), the method proceeds to stage (5)
CA 03094945 2020-3
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in order to enhance corrosion protective effect, and the
urea manufacture is continued while the supply amount of
air (oxygen) in the raw material carbon dioxide is
increased. In stage (4), if the method proceeds to stage
(5) and the supply amount of air (oxygen) in the raw
material carbon dioxide is increased, stage (6) or later
is not performed.
[0060]
In stage (6), whether to decrease the supply amount
of air (oxygen) in the raw material carbon dioxide or to
maintain the current amount is determined by evaluating
the control methods (A) to (C) as a whole. When any of
the corrosion rates determined in the control methods (A)
to (C) is equal to or less than the acceptable value but
approximate to the acceptable value (for example, more
than 95% of the acceptable value of the corrosion rate),
the method proceeds to stage (7) and the current supply
amount of air (oxygen) in the raw material carbon dioxide
is maintained. When the corrosion rates determined in
the control methods (A) to (C) are all much less than the
acceptable values (for example, equal to or less than 95%
of the acceptable values of the corrosion rates), the
method proceeds to stage (8) and the supply amount of air
(oxygen) in the raw material carbon dioxide is decreased.
[0061]
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In addition to the respective embodiments mentioned
above, the present invention also includes the following
embodiments.
[0062]
As the processing units such as the reactor 1, the
stripper 2 and the condenser 3 in the urea manufacturing
plant of the example shown in Fig. 1 are made of carbon
steel and on portions corresponding to the inner wall
surfaces, lining layers made of duplex stainless steel
are formed, the wall thicknesses cannot be measured with
ultrasonic wall thickness gauges from the outside. In
addition, as the processing units such as the reactor 1,
the stripper 2 and the condenser 3 are so high in
temperature and pressure during operation that the inside
cannot be observed either, corrosion states of the
processing units cannot be checked directly during the
operation of the urea manufacturing plant. On the other
hand, each line shown in Fig. 1 is made of the single
material stainless steel and the wall thickness can be
measured with ultrasonic wall thickness gauges from the
outside so that the corrosion state can be checked.
[0063]
Thus, during the operation of the urea manufacturing
plant shown in Fig. 1, the operation data such as the
temperatures and the pressures of the processing units
such as the reactor 1, the stripper 2 and the condenser 3
during operation and operating times thereof can be
CA 03094945 2020-3
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obtained, and along therewith, the wall thickness of each
line (the wall thicknesses of the wall thickness
measurement parts 30 to 37) can be measured and stored as
relevant data. Further, the operation of the urea
manufacturing plant shown in Fig. 1 can be ceased
periodically, and the corrosion states of the lining
layers made of duplex stainless steel inside the
processing units such as the reactor 1, the stripper 2
and the condenser 3 can be observed and stored as data.
[0064]
By comparing and evaluating the operation data such
as the temperature, the pressure and the operating time
of each processing unit, the wall thickness data of each
line, and the observation data of the corrosion state of
each processing unit with one another, the corrosion
state inside each processing unit can be estimated from
the wall thickness data of each line. This makes it
possible to estimate the corrosion state inside each
processing unit from change data of the wall thickness of
each line while continuously operating the urea
manufacturing plant. Therefore, without ceasing the
operation of the urea manufacturing plant, a replacement
timing or a maintenance timing of each processing unit
can be ascertained, and a stable urea manufacturing
operation can be performed.
[0065]
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Note that, while the present embodiment is suitable
for a case of manufacturing urea while keeping the amount
of oxygen (when air is used, air in terms of an oxygen
amount) introduced into the urea manufacturing raw
material constant instead of increasing or decreasing the
oxygen supply amount (when air is used, an air amount in
terms of an oxygen amount) as the aforementioned control
methods (A) and (B), it can be performed in combination
with one or both of the aforementioned control methods
(A) and (B).
Examples
[0066]
Example 1
Test pieces made of the stainless steels (28Cr
duplex stainless steel; S32808, and austenitic stainless
steel; S31603) were immersed respectively in urea liquids
synthesized within an autoclave. In this state, oxygen
was gradually introduced into the autoclave and oxygen
amounts when passivation films were formed on the test
pieces (Passive Corrosion) were measured. The test was
conducted at a testing temperature of 195 C. The results
are shown in Fig. 3.
[0067]
As is evident from Fig. 3, for S31603, when the
passivation film was formed (Passive Corrosion), a
corrosion portion was as slight as 0.1 mm, whereas when
CA 03094945 2020-3
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the passivation film was insufficiently formed (Active
Corrosion), the corrosion portion was much more than 10
mm. Note that it was able to be verified through the
experiment that the oxygen amount required for the
formation of the passivation film on S32808 was smaller
compared to S31603.
[0068]
From this result, it was verified that, in the urea
manufacturing method of the present invention, the
corrosion rate was able to be controlled as follows: a
passivation film was formed on the inner wall surfaces of
the plurality of processing units and the plurality of
lines constituting the urea plant shown in Fig. 1;
concentrations of iron, chromium and nickel dissolved in
urea or ammonia and an operating temperature were
measured; and a supply amount of oxygen was adjusted in
response to measurement values of the concentrations and
the operating temperature. Further, as it is a well-
known fact that, in the urea manufacturing process, a
large oxygen amount (air amount) reduces a reaction yield
of urea, it was verified that, by adjusting the supply
amount of oxygen, not only the corrosion rate, but also
the reaction yield of urea was able to be controlled.
[0069]
Example 2
During the process of manufacturing urea according
to the manufacturing flow of the urea manufacturing plant
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shown in Fig. 1, the following control methods (A), (B)
and (C) were performed.
[0070]
Control method (A)
60 days after start of urea manufacturing operation,
the wall thickness (at the wall thickness measurement
part 35) of the returned gas line 14 (an initial wall
thickness: 23.01 mm) made of S31603 series general-
purpose stainless steel (austenitic stainless steel)
connecting the stripper 2 and the condenser 3 was
measured with an ultrasonic wall thickness gauge (an
ultrasonic thickness gauge of GE Sensing & Inspection
Technologies Co., Ltd., a downsized, simply operated and
high-performance ultrasonic thickness gauge DM5E series).
A corrosion rate determined from a difference between the
measured wall thickness and the initial wall thickness
and the elapsed time was 0.12 mm/year. During a period
from the start of operation to the time of measuring, a
concentration of oxygen supplied into the raw material
carbon dioxide had been 5500 ppm and an operating
temperature (the average value) had been 183 C.
[0071]
Based on the obtained corrosion rate, it was
determined that a passivation film had been formed on the
inner wall surface of the returned gas line 14. This
means, in the embodiment shown in Fig. 2, that stage (2)
is "Yes". Accordingly, the method proceeds to stage (3).
CA 03094945 2020-09-23
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[0072]
Control method (B)
An iron concentration in a solution at an outlet of
the stripper 2 (the sampling position 41) was 0.8 ppm and
an operating temperature at that time was 171 C. Based
on the obtained iron concentration, it was determined
that a passivation film had been formed on the respective
inner wall surfaces of the reactor 1, the gas-liquid
mixture line 12 and the stripper 2 located upstream of
the sampling position 41. This means, in the embodiment
shown in Fig. 2, that stage (3) is "Yes". Accordingly,
the method proceeds to stage (4).
[0073]
Control method (C)
Operating temperatures and operating pressures of
measurement parts 51 to 53 were as follows:
measurement part 51: a temperature of 186 C, a
pressure of 151 kg/cm2G;
measurement part 52: a temperature of 188 C, a
pressure of 151 kg/cm2G; and
measurement part 53: a temperature of 180 C, a
pressure of 151 kg/cm2G.
[0074]
A flow rate of carbon dioxide (measured at the
carbon dioxide supply lines 11 and 11a) was 45000 Nm3/h.
An oxygen amount in the raw material carbon dioxide was
250 Nm3/h (calculated from an air amount introduced into
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the compressor). A flow rate of ammonia (measured at the
ammonia supply line 10) was 69 t/h. Based on data
including the measurement results above and the corrosion
rate in the control method (A), a corrosion rate of each
unit and each line was calculated as follows.
[0075]
(i) the condenser 3 (the inner wall surface is made
of S31603 series general-purpose stainless steel): 0.09
mm/year, temperature (180 C)
(ii) the stripper 2 (the inner wall surface is made
of duplex stainless steel): 0.10 mm/year, temperature
(188 C)
(iii) the reactor 1 (the inner wall surface is made
of S31603 series general-purpose stainless steel): 0.14
mm/year, temperature (186 C)
(iv) the returned gas line 14 from the stripper 2 to
the condenser 3 (the inner wall surface is made of S31603
series general-purpose stainless steel): 0.16 mm/year,
temperature (188 C)
(v) the down pipe 15 from the condenser 3 to the
reactor 1 (the inner wall surface is made of S31603
series general-purpose stainless steel): 0.09 mm/year,
temperature (180 C)
(vi) the gas-liquid mixture line 12 from the reactor
1 to the stripper 2 (the inner wall surface is made of
S31603 series general-purpose stainless steel): 0.14
mm/year, temperature (186 C)
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In any of (i) to (vi), a concentration of oxygen
supplied into the raw material carbon dioxide was 5525
ppm, and based on the obtained corrosion rates, it was
determined that a passivation film had been formed on the
inner surfaces of the respective units and those of the
respective lines. This means, in the embodiment shown in
Fig. 2, that stage (4) is "Yes". Accordingly, the method
proceeds to stage (6). Consequently, it was determined
that the corrosion rates were less than the acceptable
values and the oxygen amount was able to be decreased,
and the oxygen concentration in the raw material carbon
dioxide was decreased to 4500 ppm (stage (6) -*stage (8)
shown in Fig. 2).
Industrial Applicability
[0076]
The urea manufacturing method of the present
invention is capable of, when manufacturing urea by using
publicly-known urea manufacturing plants, manufacturing
urea in a reaction yield-efficient manner while extending
lifetimes of the plants. Therefore, it can be applied as
a manufacturing method capable of reducing a plant
operating cost and a urea manufacturing cost.
Description of Reference Numerals
[0077]
1 reactor
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2 stripper
3 condenser
heat exchanger
6 ejector
30-37 wall thickness measurement parts
40-42 sampling positions
51-56 temperature measurement parts
